In electronic packaging, a chip carrier is typically coupled to a circuit card by a solder interconnect structure that includes a grid array such as a ball grid array (BGA) or a column grid array (CGA). The solder interconnect structure traditionally contained lead. However, a need exists in the elections industry and elsewhere to eliminate the use of lead due to imminent environmental requirements in many countries. The result is a need to develop lead-free solders that can perform as well as the old lead-tin ones. The new alloys making their way into development and manufacturing are all tin-based, and in many cases alloyed with small amounts of copper, silver and/or bismuth.
One essential difference between the lead-free alloys and the lead-tin ones is that lead-free, tin-based alloys are susceptible to the formation of the so-called “tin pest”. This is a phenomenon of very sluggish phase transformation, in which the room temperature beta-tin phase is converted, below 13° C., into alpha-tin. The transformation is accompanied by massive expansion and cracking, and eventually by crumbling. The transformation is extremely slow near 13° C., but it attains a maximum rate around −30° C. to −40° C., where it can be completed within days or weeks. Since it is controlled by nucleation, the process rate is quite unpredictable; however, it certainly constitutes a great risk to any electronic devise that may, over their lifetime, be stored or operated in cold environments. Some alloying metals, such as Bi, Pb and Sb (not Cu and Ag, however), are very effective at suppressing the beta-to-alpha transformation.
Plating is a major manufacturing method of producing C4-type solder balls. While alloy plating of lead-tin alloys is well developed, commercial use of lead-free alloys plating is still in its infancy. Tin alloy baths have exhibited substantial control and aging problems that became exacerbated as additional alloying components such as bismuth are added.
Adding bismuth in a separate step through a bismuth plating method has tended to lead to bismuth deposits that are powdery and very poorly adherent to tin alloys. Accordingly, bismuth deposit can be rinsed or blown away when the structure is removed from the plating bath during routine rinsing and drying operations.
Also, adding bismuth to the solder alloy in a separate plating step is an appealing way to overcome the difficulties of alloy plating. In particular, immersion plating is in principle a desirable process because it is faster, often easier to control, and less expensive process compared to electrodeposition, since many wafers can be processed in one batch and tooling is comparatively simple. However, immersion plating is self-limiting, which is a disadvantage when a relatively thick deposit is needed for the purpose of incorporation into a reflowed connector of specified composition. For example, see Djokic, Electroless Deposition of Metals, Chapter 2, pp 54-55. Modern Aspects of Electrochemistry, No. 35, Kluwer Academics, 2002, which discusses problems with prior immersion or displacement deposition. In particular, such states that the “displacement reaction stops immediately after the reduced metal (more positive metal) covers the surface of the immersed metal (more negative metal). Accordingly, the thickness of the deposited metal is always limited. The time of immersion is particularly critical for achieving a uniform coating layer. Very often, the adhesion of the deposited films is not as good as that of films prepared by electrodeposition or by autocatalytic deposition.”
Furthermore, immersion plating often leads to dendritic growth and to porous deposits with mediocre adhesion. This specifically is the case with immersion plating of bismuth on tin out of aqueous acidified bismuth salt solutions. The resulting deposits are easily rubbed off and can even be partly detached by vigorous rinsing. More importantly, since bismuth is prone to hydrolysis in water, the large area of the porous deposit contains a substantial amount of hydroxide, which interferes with solder reflow.